This invention relates generally to heliothermal systems, and more particularly to a solar energy collector for heating fluid passing therethrough.
With the growing scarcity of fossil fuels and their sharply rising cost, it is becoming ever more apparent that a pressing need exists for an inexpensive and abundant energy source. The inexhaustible nature of solar energy and its significant magnitude over small collection areas is such as to encourage its exploitation, even at low efficiencies of recovery. Despite variations encountered in solar radiation in the course of a day, this energy can be used in a heliothermal process in which the incident radiation is absorbed and converted into heat for heating air or water to moderate temperature levels for house heating and domestic hot water.
For the typical home-owner with limited funds, the most immediate concern is the initial cost of an investment in a solar collector. Though collectors are available that are reasonably efficient and reflect a high order of technology, in some instances the price of such collectors puts them beyond the reach of the ordinary householder. Other important criteria are simplicity and durability, for the typical home-owner usually functions as his own handyman and cannot afford to engage professional assistance to maintain a sophisticated solar collector.
Hence the ideal domestic solar collector is a durable, low-cost structure of uncomplicated design which is easily maintained and yet is characterized by a high order of operating efficiency. Because solar collectors of the type heretofore available for domestic applications fall far short of this ideal, their use has been quite limited, and homes, by-and-large, continue to be heated with expensive air-polluting fossil fuel.
In solar collectors, incoming solar radiation is received through a transparent cover plate and is intercepted by a heat exchanger which absorbs heat and transfers heat to a fluid. This cover, which is formed of glass or clear plastic material, plays a vital role in determining the efficiency of the collector. Solar energy reaches the earth as electromagnetic radiation in the wavelength band between 0.3 and 3.0.mu., with its peak spectral intensity near 0.5.mu.. When solar radiation falls on a transparent cover, a part of the energy is transmitted, a part absorbed, and the remainder is reflected. These solar-optical properties depend on the wavelength, the incident angle and the composition of the cover plate.
For clear glass, the solar transmittance at an angle of incidence of 0.degree. is about 0.90, but the transmittance for long-wave radiation (5.mu.) is virtually zero. The transmittance falls and the reflection rises as the incident angle increases. Hence a glass cover plate acts as a heat trap by admitting solar radiation freely, but retaining most of the absorbed sunshine.
The efficiency of a solar energy collector is therefore partly a function of the solar-optical properties of the cover plate. To attain a high operating efficiency, the cover plate design must be such as to optimize the amount of solar radiation in the visible light range which is permitted to enter the collector and to minimize the amount of absorbed energy in the form of infra-red or heat radiation that is permitted to leak out through the same cover plate. Heat is lost through the cover plate by a combination of radiation, convection and thermal conduction.
It is well known that in order to cut down the heat loss from a solar collector, one may use a cover assembly formed by several transparent plates rather than a single panel. Thus in the Telkes U.S. Pat. No. 2,595,905, the cover is constituted by a pair of glass plates having a dead air space therebetween, the assembly admitting solar energy to the collector but acting as an effective heat insulator to prevent the outward escape of heat by radiation, conduction and convection. The Telkes patent points out that more than two transparent plates may be used for the same purpose.
The disadvantage of a multi-layer cover is that the reduction in heat loss effected thereby is accompanied by a decrease of incoming radiation, for each layer of the cover acts to reduce incoming visible energy by 10 to 15%. However, in designing a solar collector cover for a specified fixed operating condition, it is a relatively easy matter to determine the number of cover layers which afford the optimum trade-off between heat loss and optical transmission.
Another factor that significantly affects the performance of a solar collector is the quantity of heat exchange surface inside the collector. Opaque materials absorb or reflect all the incident sunshine. The absorptance .alpha. for solar radiation, and the emittance .epsilon. for long-wave radiation at the temperature of the receiving surface are particularly important in heliotechnology. For a true black body, the absorptance and emittance are equal and do not change with wavelength. But most real surfaces have heat reflectances and absorptances which may vary with wavelength. Solar collectors benefit from a high .alpha./.epsilon. ratio, while surfaces which should remain cool, such as rooftops, should have low ratios, since the objective usually is to absorb as little solar radiation and emit as much long-wave radiation as possible. Aluminum foil has a consistently low absorptance and a high reflectance over the entire spectrum from 0.25 to 25.mu., whereas black paint has a high absorptance and a low reflectance.
The heat exchange surface within the solar collector is typically a black-surfaced metal which transfers heat to the operating fluid, usually water or air. When the temperature of the operating fluid is much below the temperature of the heat exchange surface, heat is transferred very rapidly to the fluid, but as the temperature of the fluid rises and approaches equilibrium with that of the heat exchange surface, the rate of exchange slows down. In order, therefore, to effect a greater rate of heat exchange, a larger quantity of heat exchange surface is required. For specified operating conditions of a solar collector wherein the fluid input temperature, the flow rate and the internal collector temperature have known fixed values, one can readily calculate the required heat exchange area.
There are two reasons why the correct design of the metal heat exchange area is important. First, the area of heat exchange must be sufficient to effect maximum transfer of heat from the metal to the operating fluid so that the heat can be fully exploited. Second, the more heat that is transferred to the fluid, the cooler the collector runs, and the cooler the collector, the lesser is the amount of heat that is radiated toward the cover plate. By reducing the amount of radiated heat, one minimizes the heat loss from the system and thereby enhances the operating efficiency of the collector.
Thus in the theoretical case of a solar collector of conventional design operating under fixed, steady state conditions, it is a relatively simple matter to determine the number of cover plates and the area of heat exchange surface required to achieve optimum performance characteristics. But in a real situation where the motion of the sun and the vagaries of the weather represent complex variables and where practical considerations dictate the use of a bank of solar energy collectors hooked in series or in parallel relation, the actual performance of these collectors is often greatly at variance with predictions based on theoretical calculations.
In dealing with heliothermal systems, one must draw a distinction between economic and operating efficiency. The operating efficiency of any thermal energy conversion system, including a solar energy collector, is determined by the ratio of heat obtainable to useful heat received. But economic efficiency is measured by heat units obtainable per unit time and per dollar invested. Because low-level free energy is obtainable in great abundance within a relatively small area, one may tolerate a low operating efficiency if the system is inexpensive. Obviously, the desideratum is a system of the lowest possible cost and of the highest possible efficiency. In evaluating the comparative efficiency of solar energy systems, the most practical system is the one that generates the greatest useful energy output per dollar invested.